In optical communication systems, the light generated by a laser is used to represent the digital data bits “1” and “0”. Normally, a pulse of light represents “1” and the absence of light represents “0”. The bit rate is the speed at which these bits are transmitted. The temporal width of the light pulses must not exceed the bit time interval to avoid ISI (inter symbol interference).
Fibre dispersion is a phenomenon in which the spectral components of the light propagate through the fibre at different velocities due to the wavelength dependent refractive index. The effect on a pulse of light is a spreading in time that may lead to exceed the bit rate and thus to ISI (inter symbol interference). Unless dispersion is properly managed, dispersion can limit both the bit rate and the reach of an optical telecommunication system.
Managing dispersion comprises two steps. The first is measuring the dispersion and the second is compensating for the amount of dispersion measured. If the optical signal comprises two pulses, each one on a distinct wavelength, then each pulse will travel at different speed. Therefore, at the end of fibre, the dispersion causes:                1. a broadening of each of the pulses        2. a relative delay Δt between the two pulses carried by two different wavelength        
The delay Δt, referred to as Walk-off, is proportional to dispersion and wavelength separation Δλ:
                              D          ·          L                =                              Δ            ⁢                                                  ⁢            t                                Δ            ⁢                                                  ⁢            λ                                              (        1        )            
where D is the dispersion coefficient of the fibre (measured in ps/nm km) and L is the length of the fibre.
Typical values are Δλ=35 nm and Δt=50 ns. Therefore, the typical D·L value is about 1400 ps/nm corresponding to a fibre dispersion coefficient D=17 ps/nm/km and a fibre length L=80 km.
From the measurement of the relative delay Δt at the other end of the fibre and by knowing the wavelengths employed in the measurement (and therefore Δλ) the cumulated dispersion D·L, can be calculated by formula (1).
Using the walk-off effect as a method to measure dispersion is already known in the art as the “pulse-delay method”, such as that disclosed in U.S. Pat. No. 5,969,806, assigned to Tyco Submarine Systems Inc. This method measures differential delay between optical pulses launched at different wavelengths, using a multiple-wavelength transmitter at one end of the fibre and a photodiode and oscilloscope at the other end.
One way to implement the “pulse-delay method” is to have a bank of lasers that can be activated from a single pulse generator. A first laser wavelength is used as the reference time delay (given a relative group delay of zero) and the other wavelengths' transmission times are compared against this reference time (FOTP-168). However, this method suffers from a number of problems in deployed communication networks, namely:                requires at least two lasers and a single pulse generator;        requires a dedicated receiver and an oscilloscope;        requires operator interaction to control the process;        involves fibre disconnection/reconnection or dedicated taps and filters to connect the receiver        affects pre-existing traffic or prolongs time to go into service.        
Another method is disclosed in US Patent Publication number US2003/142293, Wight et al, for measuring dispersion of a link between two switching nodes in an optical network. The method described is complex and difficult to implement as it requires extra dedicated hardware and affects traffic performance on the network. Wight requires use of four full transponders: three transmitters and four receivers, with bidirectional transmission between the measurement nodes. This is because the timing reference cannot be maintained in the switch between lambda1 and lambda2. The consequence is that an extra wavelength is necessary first to transmit the clock reference from a location B to a location A, and then the clock reference, used at location A to time the signals of lambda1 and lambda2, needs to be retransmitted to location B in order to measure the phase difference with lambda1 and lambda2 signals. The method disclosed in Wight also requires synchronisation and communication of data between two transponders or the use of a third separate module to measure the phase difference and calculate the dispersion.
There is therefore a need to provide a simple and more efficient solution to the problem of measuring dispersion in a deployed and operational optical communication network.